The present invention relates to a photovoltaic system with several serially connected photovoltaic modules connected to an inverter.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
Photovoltaic systems of this type are known in the art. Such systems are typically constructed with a plurality of strands connected in parallel. The maximum number of strands depends on the power output of the inverter connected to the strands. Modern inverters can be designed for a DC input voltage approaching about 900 V.
It is presently customary to construct each strand of the system from eight photovoltaic modules, with each photovoltaic module having 60 photovoltaic cells. Accordingly, 480 cells are connected in series. Each cell has an open-circuit voltage of 1.5 V, which results in a strand voltage of 720 V, which is significantly less than the maximum voltage of 1000 V specified by the manufacturers. If a higher voltage is present, then the modules or the entire system may be destroyed.
During operation of the system, the open circuit voltage of the cells decreases to about 1 to 1.1 V, so that the voltage between the ends of the conventional strands is between 480 V and 510 V. To simplify the discussion, an exemplary operating voltage of 1 V is assumed for each cell in the subsequent figures, corresponding to 60 V across a single photovoltaic module with 60 cells. If the operator of the power grid, to which the photovoltaic system is connected, decide, for whatever reason (e.g., a short circuit in the feeder cable), to remove the photovoltaic system from the grid, then the voltage jumps to the aforementioned 720 V, which is non-critical for the modules and the system.
On the other hand, it would be desirable to operate the photovoltaic modules as well as the inverter under normal operating conditions at a higher voltage than 480-510 V, ideally at the allowable maximum voltage of 1000 V. However, this is not possible because then in oven-circuit situations, a voltage of about 1500 V would result in destruction of the photovoltaic modules, the inverter and the entire system. To this end, a switch arrangement for bridging PV modules is described in applicant's own, as of yet unpublished, German patent application DE 10 2009 016 159. However, a switch arrangement may malfunction and prevent the intended operations to be performed.
It would therefore be desirable and advantageous to provide an improved photovoltaic system to obviate prior art shortcomings which permits operation at a higher operating voltage, so as to ensure that if the voltage increases to an unacceptable high level for whatever reason, this does not cause destruction of the inverter.